Speedometer.



C. G. SMITH.

SPEEDOMETEH.

Patented Dec. 5, 1916.

17262377,???" M 41% O PW UNITED STATES PATENT OFFICE.

CHARLES G. SMITH, OF CAMBRIDGE, MASSACHUSETTS, ASSIGNOR OF ONE-HALF TO CHARLES F. MAOGILL, OF CAMBRIDGE, MASSACHUSETTS.

SPEEDOMETER.

Application filed December 15, 1915.

T 0 all whom it may concern:

Be it known that-I, CHARLES G. SMITH, a citizen of the United States, residing at Cambridge, in the county of Middlesex and State of Massachusetts, have invented certain new and useful Improvements in Speedometers; and I do hereby declare the following to be a full, clear, and exact description of the invention such as will enable others skilled in the art to which it appertains to make and use the same.

The present invention relates to speedometers of the magnetic type in which the speed indicating mcans is actuated by the magnetic current drag between a relatively movable magnetic armature energized by a field mag net and a conductor in which eddy currents are induced by the relative movement.

One object of the invention is to improve.

the accuracy of this type of speedometer by making the magnetic drag substantially independent of variations in the strength of the field magnet.

Another object of the invention is to minimize the undesirable eddy currents in the.

pole pieces of the field magnet.

In this type of speedometer, the magnetic field is furnished by a field magnet. In the field is located a conductor, such. for example, as a copper or aluminum disk. Located next to the conductor and in the magnetic field, is an armature having one or more poles for concentrating the magnetic lines to cause a field through the conductor having regions of greater strength than the rest of the field. A relative movement of the conductor and armature causes eddy currents to be set up in the conductor and produce a magnetic drag between the armature and conductor. The armature is usually rotated and the conductor is deflected against a spring, but the conductor may be rotated and the armature deflected against a spring, the relative movement between them being the essential for producing the eddy current drag. The strength of the magnetic drag is proportional to the relative speed of movement between the armature and conductor and is indicated by a pointer and scale which is usually calibrated to read in miles per hour. In order to render the instrument independent of variations in the strength of the field magnets and to thus improve its accuracy, the armature is made of a magnetic material having a low saturation point and Specification of Letters Patent.

Patented Dec. 5, 1916.

Serial No. 67,085.

the strength of the field magnets is such that the armature is worked above its saturation point. The relative excess in the number of lines of force concentrated by the poles of the armature over the rest of the magnetic field is dependent on the intensity of' the magnetization of the armature poles and not upon the absolute strength of the field in which the armature is placed. Consequently, if the strength of the field in which the armature is placed is sufiicient to bring the magnetic material of the armature up to or above the saturation point, then further increase or variation in the field will not affect the relative concentration of the lines of force by the armature and the accuracy of the instrument is not affected by such variations of the field strength.

If the armature is such as to cause a concentration of lines of force not only at the conductor but also at the poles of the field magnets, the movement of the armature relative to the field magnets will cause undesirable eddy currents to be set up in the poles of the field magnet poles. To obviate this, the armature is so constructed that there is a substantially uniform distribution of the lines of force across the spaces between the armature and the pole pieces of the field magnet.

The preferred embodiment of the invention is illustrated in the accompanying drawings in which Figure 1 is a vertical section through the speedometer; Fig. 2 is a perspective view of the upper half of the armature; Fig. 3 is a vertical section through one of the concentrating pole pieces of the armature; and Fig. 4: illustrates the curve of the relation between the magnetization and field for nickel.

Referring to the drawings, the speedometer has a permanent field magnet 1 having north and south poles N and S. The poles are conical to make a strong field across the gap between them. Located in this gap is a conductor consisting of a copper or aluminum disk 2 mounted on a supporting spindle 3 which can turn in the bearings 4. spiral spring 5 is connected to the spindle 8 and acts as a restoring force against which the magentic drag on the disk acts. A pointer 6 moving over a scale 7 indicates the deflection of the disk 2 against the restoring force of the spring.

The revolving armature 10, the movement of which causes the niagnetic drag on the conductor 2, is made up of two halves or blocks 11 and 12 of a magnetic material having a. low saturation point, such for example, as nickel. The blocks 11 and 12 are joined by yokcs 13. The upper block 11. has a. central hole 14. for the passage of the stem 13 which supports the eoi'iducting disk 2. The armature 10 is supported on a rotary shaft 15 turned by suitable mechanisn'i con nected with one of the wheels of the vehicle. One of the halves of the armature is illustrated in Fig. 2. It consists of a diffusing plate 20 and concentrating poles 21. The magnetic field between the pole pieces N and S of the permanent magnet magnetizes the armature 10. The poles 21 of the armature cause a concentration of the magnetic lines so that, as the armature is rotated, eddy can rents are set up in the conductor 2 tending to drag the conductor around against the force of the spring 5. The amount of drag and consequent movement of the pointer (5 is proportional to the speed of. the armature within the limits for which the instrument is designed.

The generation of eddy currents in the conducting disk 2 and consequent magnetic drag exerted thereon, is caused by the real or apparent movement of an unequally distributed magnetic field with relation to the conducting disk. The strength of the magnetic drag is determined by the relatively greater number of lines of force issuing from the concentrating poles 21 of the armature as compared with the number of lines of force in the regions 22 between such poles. It is not the absolute number of lines of force through the conducting disk but rather the relatively greater concentration at some regions which determine the magnetic drag. If the relative excess of the number of lines of force passing through the conducting disk 2 from the concentrating poles 21 of the armature is kept constant with relation to the regions 22 between such poles, then the amount of drag is independent of the absolute number of lines of force. The total number of lines of force passing through a given area at the face of one of the concentrating poles 21 of the armature may be expressed as Where B is the total induction, H is the magnetizing field and I is the intensity of magnetization of the material of the pole 21. When the material of the armature 21 is saturated, I reaches a constant maximum beyond which it does not go even if H is increased to as high a value as possible. The concentration 0 lines of force by the coneentrating poles 21 of the armature as comared with the regions 22 between the poles epends on the intensity of magnetization I of the poles, because if, in the above equation, the factor H is increased by increasing the strength of the field magnet 1, the increase in ll in the regions 22 between the concentrating poles 21. is the same at the corn ccntrating poles 21. Since the magnetic drag and the consequent (leliection of the conducting disk 2 depends upon the relative number of lines of force concentrated by the poles 2.1. as con'1 )ared with the number of lines of force in the regions 22 between the poles, it is apparent that the deflection of the conducting disk 2 is dependent upon I, the intensity of magnetization, and not upon the total induction B. If the armature is made of a material having a saturation value below the value of the field produced by the field n'iagi'iet 1, the intensity of magnetization will be constant and changes in the strength of the field magnet will have no corresponding effect in the :UJCAIIHCY of the instrument.

The relation between the intensity of mag netization l and the magnetizing field H for nickel is illustrated in the curve shown in Fig. 5, which is taken from Ewing. As shown in this curve, the intensity of magnetization I is constant above a magnetizing field of about 100 gausses.

So long as the material of the armature is worked above its saturation point, changes in the strength of the field magnet 1, due to temperature, aging, etc, have a negligible effect on the accuracy of the instrument. Great care need not be taken in making the field magnets 1, such as in aging them or having them of uniform strength, provided only that the field magnets are strong enough to saturate the armature. The advantage of this in turning out field magnets cheaply is obvious, as well as the advantage of rendering the accuracy of the instrument independent of variations in the strength of the field magnet due to changes in temperature, etc, when the instrument is in use.

Referring to Fig. 3 it will be noticed that the concentrating poles 21 project from a diffusing plate 20 which has a considerable thickness. The lines of force F which are crowded into the poles 21 have a chance to spread out and diffuse themselves through the diffusing plate 20 and have a much more uniform distribution as they pass from the surface of the plate 20 to the pole face of the permanent magnet 1. Thus the eddy currents in the pole faces of the magnet 1 are greatly reduced. If the concentrating poles 21 are numerous and the distance between them across the spaces 22 is small. and of the same order of magnitude as the thickness of the difiusing plate 20, then the lines of force emerging from the concentrating plate 20 have a substantially uniform distribution and eddy currents in the pole faces are reduced to a minimum. Instead of havin a diffusing plate 20 of magnetic material between the poles 21 and the face of the magnet 1, an air gap may be employed between the concentrating poles 21 and the face of the field magnet 1, provided the air gap is sufiiciently great in comparison with the distance across the spaces between the poles 21 so that the magnetic lines have a chance to spread out and have an approximately uniform distribution when they onter the pole face of the field magnet and thus minimize the eddy currents in the pole face of the field magnets.

\Vhile the preferred embodiment of the invention has been specifically illustrated and described, it is to be understood that the present invention is not limited to the illustrated embodiment but may be embodied in other constructions Within the scope of the following claims:

1. A magnetic speedometer comprising a field magnet, an eddy current conductor located in the field of the magnet, and an armature relatively movable With respect to the conductor and magnetized by the field magnet at or above saturation.

2. A magnetic speedometer comprising a field magnet, an eddy current conductor located in the field of the magnet, and an armature relatively movable with respect to the conductor and of magnetic material having a comparatively low saturation point and magnetized at or above its saturation point by the field magnet.

3. A magnetic speedometer comprising a field magnet, an eddy current conductor located in the field of the magnet, and an armature relatively movable With respect to the conductor and comprised of magnetic material magnetized by the field magnet and having a substantially constant intensity of magnetization.

4. A magnetic speedometer comprising a field magnet, an eddy current conductor located in the field of the magnet, and an arn'iature relatively movable With respect to the conductor composed of magnetic material magnetized by the field magnet to such a point as to have a substantially constant intensity of magnetization for moderate variations in the strength of the field magnet.

5. A magnetic speedometer, comprisin a field magnet, an eddy current (JOHClL1CtOI%O- cated in the field of the magnet, and an armature having a series of poles composed of a medium magnetized at or above the saturation point and separated from one another by a medium having a constant intensity of magnetization.

6. A magnetic speedometer comprising a field magnet, an eddy current conductor located in the field of the magnet, and an armature relatively movable With respect to the conductor and having a number of concentrating poles spaced sufiiciently from the pole face of the field magnet to permit a substantially uniform distribution of the magnetic lines at such face.

7. A magnetic speedometer comprising a field magnet, an eddy current conductor located in the field of the magnet, and an armature relatively movable With respect to the conductor and having a number of concentrating poles, and a diffusing plate be tween said poles and the pole face of the field magnet.

S. A magnetic speedometer comprising a field magnet, an eddy current conductor lo cated in the field of the magnet, and an armature having a series of poles magnetized to the saturation point and separated by a medium having a lesser intensity of magnetization than the poles.

CHARLES Gr. SMITH.

Copies of this patent may be obtained for five cents each, by addressing the Commissioner of Patents, Washington, I). G. 

